"I have said on repeated occasions that the tennis calendar, especially this part of the claycourt season, is poorly thought out and not playing in Barcelona, I feel, is a consequence of this," the former world number one said in a statement posted on the tournament website (www.barcelonaopenbancosabadell.com).
Nadal was in breathtaking form as he dismissed Davis Cup team mate Fernando Verdasco 6-0 6-1 in the final of the Monte Carlo Open on Sunday, but the effort looks to have taken a heavy toll on his troublesome knees.
In the run up to Roland Garros, which begins on May 23, he is expected to play in Masters events in Rome and Madrid.
He will give a news conference in Barcelona on Tuesday at 1000 GMT.
(Writing by Mark Elkington, editing by Pritha Sarkar)